This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to steady state free precession (SSFP) MRI with increased signal bandwidth.
Magnetic resonance imaging (MRI) requires placing an object to be imaged in a static magnetic field, exciting nuclear spins in the object within the magnetic field, and then detecting signals emitted by the excited spins as they precess within the magnetic field. Through the use of magnetic gradient and phase encoding of the excited magnetization, detected signals can be spatially localized in three dimensions.
Magnetic resonance imaging (MRI) is a widely used medical imaging modality that provides excellent soft-tissue contrast with arbitrary scan-volume orientations. Unlike X-ray computed-tomography or ultrasound, whose contrast is based only the transmission or reflection properties of tissue, MRI generates contrast from a variety of physical properties of tissues including relaxation, chemical-shift, diffusion and proton density. However, the primary limitation for many clinical applications of MRI, including cardiac MRI, is an insufficient signal-to-noise ratio (SNR) and/or insufficient contrast-to-noise ratio (CNR).
Over the last decade two advances in MRI show potential to significantly address the SNR and CNR limitations. First, high-field systems, specifically at 3.0 T and higher provide nearly a factor of 2 increase in signal due to increased polarization compared with standard 1.5 T systems. Second, fast gradient systems enable balanced steady-state free-precession (SSFP) imaging, which independently provides increases on the order of 50% in SNR and 100% in CNR for cardiac imaging. Unfortunately, balanced SSFP imaging is very sensitive to resonance frequency variations, demanding a very short sequence repetition time (TR) which does not allow a sufficient imaging window for adequate spatial resolution. The effects of resonance frequency variations are more pronounced at 3.0 T compared with 1.5 T. A short TR competes with many aspects of sequence design, including maximizing spatial resolution, maximizing imaging efficiency, and reducing RF power deposition
This invention describes a simple, yet very effective approach that allows a much longer TR, and imaging window than standard balanced SSFP while maintaining a reasonable level of sensitivity to resonance frequency variations caused by susceptibility and main field inhomogeneity.